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United States Patent |
5,052,187
|
Robinson, Jr.
|
*
October 1, 1991
|
Water flow control for heat pump water heaters
Abstract
A heat pump for heating water in a condenser where the water is circulated
through the condenser by a pump controlled by the condensing temperature
of the refrigerant to return water heated to a usable temperature to the
upper end of the water tank.
Inventors:
|
Robinson, Jr.; Glen P. (1050 Mt. Paran Rd., SW., Atlanta, GA 30327)
|
[*] Notice: |
The portion of the term of this patent subsequent to September 11, 2007
has been disclaimed. |
Appl. No.:
|
528704 |
Filed:
|
May 24, 1990 |
Current U.S. Class: |
62/79; 62/175; 62/238.6; 62/238.7; 237/2B |
Intern'l Class: |
F25B 007/00 |
Field of Search: |
62/79,175,238.6,238.7
237/2 B
|
References Cited
U.S. Patent Documents
4955930 | Sep., 1990 | Robinson, Jr. | 62/79.
|
Primary Examiner: King; Lloyd L.
Attorney, Agent or Firm: Powell; B. J.
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATIONS
This application is a continuation-in-part of my co-pending application
Ser. No. 07/384,148, filed July 21, 1989, now U.S. Pat. No. 4,955,930.
Claims
What is claimed as invention is:
1. A water heater construction for storing hot water at the normal
predetermined tank temperature associated with hot water heaters
including:
a water tank having an upper end, a lower end, and a cold water supply for
supplying cold water to the lower end of said water tank;
a heat pump having a condenser heat exchanger externally of said water
tank; circulation pump means for pumping water from the lower end of said
water tank through said condenser heat exchanger to heat the water and
back into the upper end of said water tank; thermostatic control means
responsive to the temperature of the water at a predetermined position in
said water tank and operatively connected to said heat pump, said
thermostatic control means set at the normal predetermined tank
temperature to operate said heat pump when the temperature of the water in
said water tank at said predetermined position drops below the normal
predetermined tank temperature until the temperature of the water at said
predetermined position in said water tank is raised back to the normal
predetermined tank temperature; and
temperature sensing means operatively associated with the condenser heat
exchanger refrigerant temperature and operatively connected to said
circulation pump means to start operating said pump means when the
condensing refrigerant temperature reaches a first prescribed value and to
stop operating said pump means when the condensing refrigerant temperature
drops below a second prescribed value, said first and second prescribed
values selected so that water from said tank in said condenser heat
exchanger will not be discharged from said heat exchanger and returned to
the upper end of said water tank until a predetermined return water
temperature sufficient for immediate use but below the normal
predetermined tank temperature, and the discharge of the water from said
heat exchanger will be stopped when the return water temperature drops
significantly below said predetermined return water temperature so that
the water in said water tank is first intermittently circulated through
said condenser heat exchanger to heat the water to said predetermined
return water temperature in batches until substantially all of the water
in said water tank is at least at said predetermined return water
temperature and then further circulated through said condenser heat
exchanger to heat the water above said predetermined return water
temperature until the water in said water tank has been heated to the
normal predetermined tank temperature at said predetermined position.
2. The water heater construction of claim 1 wherein said prescribed
temperature corresponds to a water temperature in said heat exchanger of
about 125.degree.-140.degree. F.
3. The water heater construction of claim 2 wherein said second prescribed
temperature is about 15.degree. F. less than said first prescribed
temperature.
4. The water heater construction of claim 2 wherein said first prescribed
temperature is about 140.degree. F.
5. The water heater construction of claim 1 wherein said temperature
sensing means is a temperature responsive switch.
6. The water heater construction of claim 1 wherein the heating rate
capacity of said heat pump is insufficient to maintain the water flowing
through said heat exchanger at the normal predetermined tank temperature
at the pumping capacity of said circulation pump so that the average
temperature of the water within the heat exchanger is minimized and the
heat transfer rate from the heat pump into the water is maximized.
7. A heating unit for heating water in an existing water heater having a
water tank for storing water at the normal predetermined tank temperature
associated with the water heater into which cold water is introduced
adjacent the lower end thereof and from which hot water is withdrawn from
adjacent the upper end thereof, said heating unit comprising:
a heat pump including a refrigerant-to-water heat exchanger adapted to heat
water therein while said heat pump is operating, said heat exchanger
connected in a water loop between the lower and upper ends of said water
tank;
a circulation pump in said water loop for selectively forcing water through
said heat exchanger from the lower end of said tank to the upper and
thereof;
thermostatic control means responsive to the temperature of the water at a
predetermined position in the water tank being below the normal
predetermined tank temperature to operate said heat pump until the
temperature of the water at the predetermined position in the water tank
is raised back to the normal predetermined tank temperature; and,
temperature control means operatively associated with the refrigerant
temperature in said heat exchanger and said circulation pump to operate
said circulation pump when the refrigerant temperature in said heat
exchanger reaches a predetermined value and to operate said circulation
pump until the refrigerant temperature drops below said predetermined
value a prescribed amount.
8. The heating unit of claim 7 wherein the heating rate capacity of said
heat pump is insufficient to maintain the water flowing through said heat
exchanger at the normal predetermined tank temperature at the pumping
capacity of said circulation pump so that the average temperature of the
water within the heater exchanger is minimized and the heat transfer rate
from the heat pump into the water is maximized.
9. The water heater construction of claim 8 wherein said predetermined
value of refrigerant temperature corresponds to a water temperature in
said heat exchanger of about 125.degree.-140.degree. F.
10. A method of heating water in a water tank to the normal predetermined
tank temperature associated with water heaters using a heat pump with a
condenser heat exchanger comprising the steps of:
a) connecting the condenser heat exchanger between the upper and lower ends
of the water tank;
b) operating the heat pump when the tank temperature falls below the normal
predetermined tank temperature;
c) detecting the refrigerant temperature in the condenser heat exchanger;
d) when the refrigerant temperature in the condenser heat exchanger exceeds
a first prescribed temperature corresponding to a water temperature
sufficient for immediate use but below the normal predetermined tank
temperature, circulating the water from the lower end of the tank to the
heat exchanger and from the heat exchanger to the upper end of the tank
until the refrigerant temperature is lowered a prescribed amount;
e) stopping the circulating of the water through the heat exchanger when
the refrigerant temperature has been lowered said prescribed amount; and,
f) repeating steps d) and e) until the normal predetermined tank
temperature is reached.
11. The method of claim 10 wherein said first prescribed temperature
corresponds to a water temperature in the heat exchange of about
125.degree.-140.degree. F. and said prescribed amount the refrigerant
temperature is lowered is about 15.degree. F.
12. The method of claim 11 wherein the predetermined tank temperature is
about 140.degree. F.
13. The method of claim 10 wherein said first prescribed temperature is
about 140.degree. F.
14. The method of claim 10 wherein step b) includes heating the water in
the heat exchanger at a rate less than that required to heat the water to
the normal predetermined tank temperature while circulating through the
heat exchanger in step d).
15. The method of claim 10 wherein step d) includes circulating the water
through the heat exchanger at a flow rate such that the water temperature
in the heat exchanger drops below a water temperature sufficient for
immediate use when the water a the lower end of the tank is cold.
Description
BACKGROUND OF THE INVENTION
This invention relates generally to heating devices for water heaters; and
more particularly to a heat pump used to heat the water in a water heater.
Heat pumps have been used before to heat water for water heating
installations. Examples of these uses are illustrated in the following
United States Patents:
______________________________________
Class/
U.S. Pat. No.
Issued Inventor Subclass
______________________________________
2,575,325
11/1951 Ambrose et. al. 62/238 E
2,668,420
2/1954 Hammell 62/238 E
3,922,876
12/1975 Wetherington, Jr., et. al.
62/238 EX
4,073,285
2/1978 Wendel 62/238.6
4,136,731
1/1979 DeBoer 165/12
4,141,222
2/1979 Ritchie 62/238 E
4,142,379
3/1979 Kuklinski 62/238.6
4,330,379
5/1982 Robinson, Jr. 62/181
______________________________________
Typically early prior art heat pumps for water heaters employed thermally
operated flow control valves to restrict the rate of water flow through
the heat pump to assure that the outlet water reached a sufficiently high
temperature so that the heated water could be returned to the top of a
water storage tank and be available for immediate use. Usually, water was
drawn from the bottom of the tank through the dip tube on the cold water
inlet at the top of the tank which extends down to the lower end of the
tank. The heated water was returned to the top of the tank after being
heated. A water circulating pump associated with the heat pump was
required to provide a flow rate of approximately 2 GPM per 12,000 BTUH to
maintain sufficient heat transfer to extract the heat from the condenser
without exceeding the condensing temperature limit of the compressor as
the water approached its final tank temperature.
The prior art flow control valves were very similar to the thermostat in an
automobile radiator system. A bleed hole allowed a small amount of water
to flow through the heat pump condenser heat exchanger when the water from
the water tank was cold. This allowed the flow control valve to sense the
temperature of the water leaving the heat pump. Typically, the valve would
begin to open as the water approached about 115.degree. F. and was fully
open at about 125.degree. F. During an initial tank heat up or after a
batch of hot water was withdrawn, the flow control valve would modulate
the water flow rate to maintain an outlet temperature of approximately
120.degree. F. until the entire tank began to heat up. As the water
entering the heat pump from the bottom of the tank began to warm, the
output of the heat pump would raise the temperature of the water higher
than 120.degree. F. causing the flow control valve to open further to
increase the flow rate until the maximum flow rate was reached. The system
continued to operate until the tank was heated to its set point as
controlled by the tank thermostat, usually about 140.degree. F.
The advantage of this system was that it heated the water tank from the top
down making some hot water instantaneously available before a tank was
completely heated to an acceptable temperature. Unfortunately, this type
flow control valve experienced serious reliability problems from
corrosion, scaling, and plugging. Other types of flow control valves were
also found to either be too expensive and/or unreliable to be practical.
Because of the problems, this concept of using variable flow control was
about abandoned in the mid-1980's.
SUMMARY OF THE INVENTION
These and other problems and disadvantages associated with the prior art
are overcome by the invention disclosed herein by providing a heat pump
for heating the water in a water heater which has a high temperature hot
water recovery without requiring the use of flow control valves. This
allows the advantages associated with prior art heat pumps with flow
control to be achieved more reliably and economically.
The apparatus of the invention includes generally a water tank which
contains the water to be heated with the water in the lower level of the
tank circulated through the condenser heat exchanger in a heat pump
located externally of the water tank by a circulation means such as a pump
so that water from the bottom of the water tank can be circulated through
the condenser heat exchanger to heat the water and then back into the top
of the water tank. Refrigerant temperature operated control means is
provided for letting the water remain in the condenser heat exchanger
until the water in the condenser heat exchanger reaches a predetermined
return temperature so that the water returned to the top of the water tank
is at least at the predetermined return temperature. The predetermined
temperature is selected to be hot enough for immediate use by the user.
Because the water in the water tank naturally stratifies with the hotter
water being at the top, the hot water being returned from the condenser
heat exchanger in the heat pump can be immediately used by the user of the
invention. The control means may be a temperature operated switch which
controls the circulation pump operation. The switch is set to operate the
pump when the refrigerant temperature in the condenser reaches a value
corresponding to the desired return water temperature and to stop pump
operation when the refrigerant temperature drops a specified amount. This
discharges the water intermittently in pulses from the condenser into the
tank as each condenser full of water is heated. When the water from the
water tank into the condenser reaches a temperature where the condenser
can heat the water at least to the return temperature without stopping the
pump, the pump continues to run until the heat pump is turned off when the
desired tank temperature is reached. These and other features and
advantages of the invention will become more clearly understood upon
consideration of the following specification and accompanying drawings
wherein like characters of reference designate corresponding parts through
the several views and in which:
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 schematically illustrates the invention connected to the hot water
tank;
FIG. 2 is a temperature-time diagram of the water in the water tank using
the invention; and,
FIG. 3 is a temperature-time diagram of the water in the heat pump heat
exchanger during initial heating.
These figures in the following detailed description disclose specific
embodiments of the invention; however, the inventive concept is not
limited thereto since it may be embodied in other forms.
DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
FIG. 1 schematically illustrates the invention utilizing an existing water
heater H with electrical resistance upper and lower heating elements
H.sub.1 and H.sub.2 respectively. The resistance heating elements are
disabled while the invention is being used. The water heater H is of
conventional construction with a generally vertically oriented water tank
T having a cold water connection CWC and a hot water connection HWC both
located at the upper end of the tank T. The cold water connection CWC has
a dip tube DT that extends from the top of the tank down to a position
adjacent the bottom of the tank so that incoming cold water is delivered
to the bottom of the water tank as is conventional. The hot water
connection HWC, on the other hand, opens into the upper end of the tank T.
Because the water in the upright tank naturally stratifies according to
temperature with the hottest temperature being at the upper end, the
hottest temperature water in the tank is withdrawn through the hot water
connection HWC.
The heating unit 10 illustrated in the drawings includes a heat pump loop
11 and a water circulation loop 12. The heat pump loop 11 includes a
conventional compressor 14 with its suction side connected to an
evaporator heat exchanger 15 illustrated as an air-to-refrigerant heat
exchanger and fan and with its high pressure side connected to a condenser
heat exchanger 16 shared with the water circulation loop 12. The condenser
heat exchanger 16 is a refrigerant-to-liquid heat exchanger. The
refrigerant in the heat pump loop passes through the refrigerant side of
the condenser heat exchanger 16 while the water in the water circulation
loop 12 passes through the water side of the condenser heat exchanger 16
as will become more apparent. The refrigerant side of the condenser heat
exchanger 16 is connected to the evaporator heat exchanger 15 through the
conventional expansion device 18.
The water circulation loop 12 includes the condenser heat exchanger 16
shared with the heat pump 11 and a water pump 19. The intake pipe 20 to
the water circulation loop 12 is connected to the cold water connection
CWC through a tee fitting 21 which also serves to connect the cold water
supply pipe CWP to the cold water connection CWC. Similarly, the discharge
pipe 22 from the water circulation loop 12 is connected to the hot water
connection HWC through a tee fitting 24. The tee fitting 24 also serves to
connect the hot water supply pipe HWP to the hot water connection HWC. As
will become more apparent, these connections permit the cold water from
the cold water supply pipe CWP to enter the tank as hot water is drawn
off, while at the same time allowing the water circulation loop 12 to
withdraw the cold water from the bottom of the tank. Similarly, hot water
is drawn out of the top of the tank through the connection HWC and the
heated water from the water circulation loop 12 is returned to the top of
the tank through the same connection.
The overall operation of the heating unit 10 is controlled by a tank
thermostat 25 located so as the sense tank water temperature adjacent the
lower end thereof. Thermostat 25 may be the conventional lower thermostat
associated with the heating elements H.sub.1 and H.sub.2 in a conventional
electric water heater or may be a separate thermostat. The thermostat 25
is typically designed to open when the tank water temperature at its
location reaches the set point of the thermostat and will close when the
tank water temperature drops a prescribed amount below the set point
temperature. The set point temperature for the thermostat 25 is usually
lower than the final temperature of the water at the top of the tank since
the water stratifies. The operation of the water circulation pump 19 is
controlled by a refrigerant temperature switch 26 connected to the heat
pump loop 11 so as to sense refrigerant condensing temperature.
Temperature switch 25 has a configuration to close when the refrigerant
condenser temperature reaches a preset value and opens when the condenser
temperature drops a predetermined value below its preset point.
The maximum water temperature to which the heating unit 10 can heat the
water is established by the maximum safe condensing temperature at which
the heat pump compressor of loop 11 can operate when full condensing of
the refrigerant of the condenser heat exchanger 16 takes place. Usually, a
refrigerant such as Refrigerant R500 normally used in water heating
applications use compressors which reliably operate at a condensing
temperature of approximately 140.degree. F. The minimum temperature at
which water can be returned to the top of the water tank and be ready for
immediate use is established by typical use requirements and is typically
in the neighborhood of about 110.degree.-125.degree. F. Thus, the heating
unit 10 can be operated until the water at the upper end of the tank is
about 140.degree. F. The temperature switch 26 illustrated is selected so
that it closes to operate the water pump 19 when the refrigerant
condensing temperature reaches about 140.degree. F. and opens when the
condensing temperature falls to about 125.degree. F. to stop the operation
of the pump 19.
OPERATION
Usually the water in the tank T is heated so that the selected water
temperature is maintained at the level set by the tank thermostat 25. A
typical setting is about 130.degree. F. Because the water in the tank T
tends to stratify, there will usually be a temperature gradient between
the upper end of the tank T and the level of the thermostat 25 so that the
temperature of the water in the upper level of the tank T is at a
temperature of about 140.degree. F.
When the user opens a tap for hot water, the hotter water at the upper end
of the tank T is drawn off while fresh cold water from the supply pipe CWP
enters the lower end of tank T. Because heated water stratifies extremely
well if there is no agitation to cause the mixing with the cold water, the
cold water remains in the lower end of the tank T. As soon as the cold
water level reaches the vicinity of the tank thermostat 25 so that the
temperature drops below the setting of the thermostat 25, it closes to
start operation of the heating unit 10.
Closing of thermostat 25 starts the compressor 14 to supply heated
refrigerant to the refrigerant side of the condenser heat exchanger 16. It
will be appreciated that the water side of the condenser heat exchanger 16
always remains connected to the water tank and remains full of water. When
the compressor is initially turned on, the heat exchanger 16 is cool. This
cool coil causes the condensing temperature on the refrigerant side of the
heat exchanger 16 to be low. The temperature switch 26 remains open,
however, since the refrigerant temperature is below the set point of the
temperature switch 26. This prevents the pump from operating to circulate
water from the tank T. The water temperature differentials and the pipe
sizes associated with the heating unit 10 are such that very little water
flow occurs through the water circulation loop due to a thermosiphon
affect and remains virtually stagnant until the water circulation pump 19
is operating.
The stagnant water in the condenser heat exchanger 16 will be heated as the
hot refrigerant continues to flow through the exchanger by absorbing the
heat output of the compressor. This causes the condensing temperature to
increase rapidly. When the set point temperature of the temperature switch
26 is reached, the switch will close to operate the water pump 19.
Typically, the set point temperature is about 140.degree. F. with
Refrigerant R500.
As soon as pump 19 starts to operate, the heated water is discharged into
the top of the tank T and is replaced by cold water from the bottom of the
tank. As the cold water starts to fill the heat exchanger 16, the
temperature and thus the condensing temperature on the refrigerant side
rapidly falls until it reaches the lower temperature differential
permitted from the preset point. In the particular example used, this
temperature differential is about 15.degree. F. Thus, when the condensing
temperature falls to about 125.degree. F., the temperature switch 26 will
open to disable the water pump 19.
As the heat pump loop 11 continues to operate, the water pump 19 will be
pulsed on and off each time the water in the condenser heat exchanger 16
is heated up to the point where the condensing temperature reaches the set
point of the temperature switch 26. FIG. 3 illustrates this phenomenon.
Because the water in the condenser heat exchanger 16 is quickly heated,
the pump 19 will be frequently pulsed on and off during the heating cycle.
In the particular example illustrated, it takes about 20 seconds for the
water in the heat exchanger 16 to heat from about 60.degree. F. to
140.degree. F. and about 3 seconds for the pump to discharge enough of the
water from the heat exchanger 16 and introduce cold water from the tank T
to reduce the condensing temperature and cause the cycle to repeat. As
will become more apparent, it will be seen that these short pulse cycles
will continue until the water at the lower end of the tank starts to heat
up from the initial cold temperature to a temperature displaced below the
lowest temperature at which the temperature switch 26 keeps the pump 19
operating. Typically this is about 5.degree.-10.degree. F. below the lower
condensing temperature at which the temperature switch 26 opens.
Because heated water stratifies extremely well when there is no agitation
to cause the mixing with cold water, there will be a distinct boundary
between the hot and cold water in the tank T as hot water is drawn off the
top of the tank and cold water is supplied to the bottom of the tank.
Likewise, when the cold water is drawn from the bottom of the tank, heated
in the heating unit 10 and returned to the top of the tank, the boundary
or thermocline will remain between the hot and the cold water. This
thermocline slowly moves downwardly in the tank as heating progresses.
FIG. 2 illustrates the temperature of the water in various depths TC1-TC6
in the tank T as it is being heated from an initial temperature in which
the entire tank is cold. The temperature at which the hot water is
returned to the top of the tank from the heating unit 10, accounting for
typical heat losses, is about 110.degree.-120.degree. F. Because of this
stratification, the upper end of the tank is quickly heated to a usable
temperature while a much longer time is required to heat the entire tank
up to the usable temperature. The illustration used in FIG. 2 is based on
a 40 gallon water tank and a 12000 BTUH heat pump. It will thus be seen
that the upper end of the tank is heated to about 120.degree. F. in about
15 minutes while it takes about 100 minutes to heat all of the tank up to
this 120.degree. F. temperature. After the tank has reached the
120.degree. F. temperature, the water being returned from the bottom of
the tank to the condenser heat exchanger 16 is at the 120.degree. F.
temperature so that the temperature switch 26 remains closed to
continuously operate the pump 19. The pump then operates continuously
until the final tank temperature is raised to the set point on the tank
thermostat 25.
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